The way to specify timezone offsets differs between the PDF and XMP
specs, so hardcode the UTC offset instead of using `%z`. This also
avoids the possibility of embedding the more more granular timezones
supported by the datetime module but that are unsupported by either PDF
or XMP formats.
In line with the previous commit for consistency, convert the date
strings from --creationdate and --moddate to UTC as well if a timezone
is specified.